A crossover is where a story includes two or more distinct characters, settings or universes.
Swiftfire has had several crossovers with other fan fiction series. The most notable was in 2001 with the Star Trek: Unity crossover that saw several series come together to deal with a specific event, an invasion by the Vendoth and then their attack on Earth.
The incomplete "There's No Angels Here" was originally a crossover into the Star Trek: Pendragon universe. In 2013 the story was moved into a new universe inspired by Pendragon due to the Pendragon's author no longer being active. With the later decision to reboot the series it is unknown if this piece will be completed due to planned or future changes in continuity.
Back in 2001 there were also plans to create a crossover between Star Trek: The Legend Continues and SWF. However, the idea never got past the planning stages.